---
c: Copyright (C) The libssh2 project and its contributors.
SPDX-License-Identifier: BSD-3-Clause
Title: libssh2_userauth_publickey_sk
Section: 3
Source: libssh2
See-also:
  - libssh2_session_init_ex(3)
---

# NAME

libssh2_userauth_publickey_sk - authenticate a session with a FIDO2 authenticator

# SYNOPSIS

~~~c
#include <libssh2.h>

int
libssh2_userauth_publickey_sk(LIBSSH2_SESSION *session,
                              const char *username,
                              size_t username_len,
                              const unsigned char *publickeydata,
                              size_t publickeydata_len,
                              const char *privatekeydata,
                              size_t privatekeydata_len,
                              const char *passphrase,
                              LIBSSH2_USERAUTH_SK_SIGN_FUNC((*sign_callback)),
                              void **abstract);
~~~

# CALLBACK

~~~c
#define LIBSSH2_SK_PRESENCE_REQUIRED     0x01
#define LIBSSH2_SK_VERIFICATION_REQUIRED 0x04

typedef struct _LIBSSH2_SK_SIG_INFO {
    uint8_t flags;
    uint32_t counter;
    unsigned char *sig_r;
    size_t sig_r_len;
    unsigned char *sig_s;
    size_t sig_s_len;
} LIBSSH2_SK_SIG_INFO;

int name(LIBSSH2_SESSION *session, LIBSSH2_SK_SIG_INFO *sig_info,
         const unsigned char *data, size_t data_len, int algorithm,
         uint8_t flags, const char *application,
         const unsigned char *key_handle, size_t handle_len,
         void **abstract);
~~~

# DESCRIPTION

*session* - Session instance as returned by libssh2_session_init_ex(3)

*username* - Name of user to attempt authentication for.

*username_len* - Length of username parameter.

*publickeydata* - Buffer containing the contents of a public key file. If
NULL, the public key will be extracted from the privatekeydata. When using
certificate authentication, this buffer should contain the public certificate
data.

*publickeydata_len* - Length of public key data.

*privatekeydata* - Buffer containing the contents of a private key file.

*privatekeydata_len* - Length of private key data.

*passphrase* - Passphrase to use when decoding private key file.

*sign_callback* - Callback to communicate with FIDO2 authenticator.

*abstract* - User-provided data to pass to callback.

Attempt FIDO2 authentication. using either the sk-ssh-ed25519@openssh.com or
sk-ecdsa-sha2-nistp256@openssh.com key exchange algorithms.

This function is only supported when libssh2 is backed by OpenSSL.

# CALLBACK DESCRIPTION

*session* - Session instance as returned by libssh2_session_init_ex(3)

*sig_info* - Filled in by the callback with the signature and accompanying
information from the authenticator.

*data* - The data to sign.

*data_len* - The length of the data parameter.

*algorithm* - The signing algorithm to use. Possible values are
LIBSSH2_HOSTKEY_TYPE_ED25519 and LIBSSH2_HOSTKEY_TYPE_ECDSA_256.

*flags* - A bitmask specifying options for the authenticator. When
LIBSSH2_SK_PRESENCE_REQUIRED is set, the authenticator requires a touch. When
LIBSSH2_SK_VERIFICATION_REQUIRED is set, the authenticator requires a PIN.
Many servers and authenticators do not work properly when
LIBSSH2_SK_PRESENCE_REQUIRED is not set.

*application* - A user-defined string to use as the RP name for the
authenticator. Usually "ssh:".

*key_handle* - The key handle to use for the authenticator's allow list.

*handle_len* - The length of the key_handle parameter.

*abstract* - User-defined data. When a PIN is required, use this to pass in
the PIN, or a function pointer to retrieve the PIN.

The *sign_callback* is responsible for communicating with the hardware
authenticator to generate a signature. On success, the signature information
must be placed in the *sig_info* sig_info parameter and the callback must
return 0. On failure, it should return a negative number.

The fields of the LIBSSH2_SK_SIG_INFO are as follows.

*flags* - A bitmask specifying options for the authenticator. This should
be read from the authenticator and not merely copied from the flags parameter
to the callback.

*counter* - A value returned from the authenticator.

*sig_r* - For Ed25519 signatures, this contains the entire signature, as
returned directly from the authenticator. For ECDSA signatures, this contains
the r component of the signature in a big-endian binary representation. For
both algorithms, use LIBSSH2_ALLOC to allocate memory. It will be freed by the
caller.

*sig_r_len* - The length of the sig_r parameter.

*sig_s* - For ECDSA signatures, this contains the s component of the
signature in a big-endian binary representation. Use LIBSSH2_ALLOC to allocate
memory. It will be freed by the caller. For Ed25519 signatures, set this to
NULL.

*sig_s_len* - The length of the sig_s parameter.

# RETURN VALUE

Return 0 on success or negative on failure. It returns
LIBSSH2_ERROR_EAGAIN when it would otherwise block. While
LIBSSH2_ERROR_EAGAIN is a negative number, it is not really a failure per se.

# ERRORS

Some of the errors this function may return include:

*LIBSSH2_ERROR_ALLOC* - An internal memory allocation call failed.

*LIBSSH2_ERROR_SOCKET_SEND* - Unable to send data on socket.

*LIBSSH2_ERROR_AUTHENTICATION_FAILED* - failed, invalid username/key.

# AVAILABILITY

Added in libssh2 1.10.0